UNIVERSITY  OF  CALIFORNIA  PUBLICATIONS 

COLLEGE  OF  AGRICULTURE 
AGRICULTURAL  EXPERIMENT  STATION 

BERKELEY,  CALIFORNIA 


EXPERIMENTS  WITH  STOCKS 
FOR  CITRUS 


BY 

W.  W.  BONNS  and  W.  M.  MERTZ 


BULLETIN  No.  267 

Berkeley,  Cal.,  March,  1916 


UNIVERSITY  OF  CALIFORNIA   PRESS 
BERKELEY 
1916 


Benjamin  Ide  Wheeler,  President  of  the  University. 

EXPERIMENT  STATION  STAFF 

HEADS   OF  DIVISIONS 

Thomas  Forsyth  Hunt,  Director. 

Edward  J.  Wickson,  Horticulture  (Emeritus). 

Herbert  J.  Webber,  Director  Citrus  Experiment  Station;  Plant  Breeding. 

Hubert  E.  Van  Norman,  Vice-Director;  Dairy  Management. 

William  A.  Setchell,  Botany. 

Myer  E.  Jaffa,  Nutrition. 

Robert  H.  Loughridge,  Soil  Chemistry  and  Physics  (Emeritus). 

Charles  W.  Woodworth,  Entomology. 

Ralph  E.  Smith,  Plant  Pathology. 

J.  Eliot  Coit,  Citriculture. 

John  W.  Gilmore,  Agronomy. 

Charles  F.  Shaw,  Soil  Technology. 

John  W.  Gregg,  Landscape  Gardening  and  Floriculture. 

Frederic  T.  Bioletti,  Viticulture  and  Enology. 

Warren  T.  Clarke,  Agricultural  Extension. 

John  S.  Burd,  Agricultural  Chemistry. 

Charles  B.  Lipman,  Soil  Chemistry  and  Bacteriology. 

Clarence  M.  Haring,  Veterinary  Science  and  Bacteriology. 

Ernest  B.  Babcock,  Genetics. 

Gordon  H.  True,  Animal  Husbandry. 

James  T.  Barrett,  Plant  Pathology. 

Fritz  W.  Woll,  Animal  Nutrition. 

A.  V.  Stubenrauch,  Pomology. 

Walter  Mulford,  Forestry. 

W.  P.  Kelley,  Agricultural  Chemistry. 

H.  J.  Quayle,  Entomology. 

Elwood  Mead,  Rural  Institutions. 

J.  B.  Davidson,  Agricultural  Engineering. 

H.  S.  Reed,  Plant  Physiology. 

D.  T.  Mason,  Forestry. 

William  G.  Hummel,  Agricultural  Education. 

Leon  M.  Davis,  Dairy  Industry. 

John  E.  Dougherty,  Poultry  Husbandry. 

S.  S.  Rogers,  Olericulture. 

*  Frank  Adams,  Experimental  Irrigation. 

David  N.  Morgan,  Assistant  to  the  Director. 

Mrs.  D.  L.  Bunnell,  Librarian. 

CITRUS   EXPERIMENT   STATION 
POMOLOGY 

W.   W.  Bonns.  W.   M.   Mertz. 


*  In  co-operation  with  office  of  Public  Roads  and  Rural  Engineering,  U.  S.  Department  of 
Agriculture. 


EXPERIMENTS  WITH  STOCKS  FOR  CITRUS1 

By  W.  W.  BONNS  and  W.  M.  MERTZ 


INTRODUCTION 

Citriculture,  like  other  branches  of  agricultural  art  dealing  with 
the  production  of  tree  fruits,  is  fundamentally  concerned  with  certain 
problems  of  plant  propagation.  Fruit  varieties  of  desirable  qualities 
and  commercial  value  must  be  perpetuated ;  why  this  end  cannot  gen- 
erally be  attained  through  the  production  of  seed  from  cross-pollinated 
fruits,  the  Mendelian  discovery  has  only  recently  explained.  Practical 
attainment,  however,  preceded  the  discovery  by  centuries,  and  the  arts 
of  budding  and  grafting  have  preserved  the  desired  fruit  varieties 
through  succeeding  generations. 

The  use  of  such  asexual  methods  of  propagation  introduced  into 
horticulture  the  scientific  problem  of  the  relation  and  interaction  of 
stock  and  scion.  The  literature  of  horticulture  and  the  sciences  bear- 
ing thereon  are  full  of  observations  and  deductions  relating  to  this 
question.  Much  of  the  early  work  has  little  scientific  weight ;  later  in- 
vestigations, chiefly  in  the  field  of  botany,  have  attacked  the  problem 
in  a  truly  scientific  manner,  with  results  of  great  interest  and  signifi- 
cance. A  survey  of  the  literature,  however,  at  least  in  so  far  as  it 
relates  to  tree  fruits,  leads  to  the  conclusion  that  little  specific  informa- 
tion based  upon  definite,  careful  experiments  with  well-defined  condi- 
tions, is  available.1  Especially  noticeable  is  the  paucity  of  orchard 
experiments  of  sufficient  size. 

The  very  evident  importance  of  definite  knowledge  regarding  the 
various  stocks  for  citrus  and  the  relations  of  the  stock  to  the  several 
species  and  varieties  grown  for  commerce  in  regard  to  optimum  growth 
and  production,  should  make  emphasis  of  the  subject  unnecessary.  As 
a  matter  of  fact,  there  is  probably  no  factor  connected  with  his  business 
that  has  received  less  thought  from  the  average  California  citrus 
grower.  The  phenomenal  growth  of  the  industry,  its  beginnings  in 
the  profitable  sweet  seedling  groves,  and  the  subsequent  great  demand 
for  nursery  stock,  has  resulted  in  an  almost  universal  use  of  two  roots 


*  Paper  No.  11,  Citrus  Experiment  Station,  College  of  Agriculture,  University 
of  California,  Eiverside,  California. 

i  For  a  review  of  this  subject  see  Prof.  U.-P.  Hedrick's  article  "Stocks  for 
Fruit  Trees,"  Monthly  Bulletin,  California  State  Commission  of  Horticulture, 
Vol.  3,  pp.  449-455  (1914). 


276  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

— sweet  orange  (Citrus  sinensis)  and  sour  orange  (Citrus  aurantium) . 
Little  consideration  has  been  given  to  stocks  in  relation  to  differences 
in  the  environmental  factors  of  soil,  soil  moisture,  temperature,  and 
humidity.  Indeed,  it  may  well  be  doubted  if  fifty  per  cent  of  the 
owners  of  California  citrus  groves  have  certain  knowledge  of  the  stock 
upon  which  their  trees  are  growing. 

Lack  of  such  knowledge  may  seem  of  small  moment  to  owners  of 
trees  of  productive  age.  To  prospective  buyers,  to  persons  setting  out 
new  acreage,  and  to  nurserymen  the  question  of  stocks  should  be  rated 
at  its  proper  importance.  Productiveness  may  be  as  dependent  upon 
the  nature  and  vigor  of  the  stock  used  and  its  adaptability  to  soil 
environment  as  to  any  characters  inherent  in  the  buds  grown  upon 
such  stock. 

As  illustrative  of  the  practical  bearing  of  the  matter  on  the  future 
success  of  the  citrus  grove  may  be  cited  examples  of  the  variation  in 
disease  resistance  of  different  stocks. 

In  the  autumn  of  1914  members  of  the  Station  staff  inspected  a 
nursery  in  southern  California  where  sweet  and  sour  orange  stocks 
were  being  grown.  An  examination  of  one  thousand  trees  of  each  kind 
showed  29  per  cent  of  the  sweet  stock  affected  to  some  degree  with 
gum  disease.  Not  a  single  case  of  the  malady  was  found  among  the 
sour  stock.  This  evident  difference  in  disease  resistance  under  natural 
conditions  supports  the  findings  of  Fawcett2  that  sour  stock,  artifically 
inoculated  with  fungi  capable  of  inducing  gummosis  has  an  inherent 
constitutional  resistance  to  the  disease1,  which  sweet  seedlings  do  not 
possess. 

In  the  spring  of  1914,  a  seed-bed  of  about  fifty  thousand  seeds  was 
planted  at  Riverside,  comprising  seed  of  sweet  orange,  sour  orange, 
trifoliate  orange,  pomelo,  and  rough  lemon.  Shortly  after  planting 
and  continuing  up  to  the  time  that  the  seedlings  were  attaining  their 
first  growth,  the  seed-bed  was  subjected  to  a  severe  attack  of  "damping 
off"  by  a  soil  fungus  of  the  genus  Rhizoctonia.  A  large  percentage  of 
the  stock  succumbed  to  this  fungous  invasion,  but  the  noteworthy  fact 
was  the  wide  variation  in  the  amount  of  injury  done  to  the  several 
blocks  of  stock. 

That  the  fungus  was  rather  uniformly  distributed  in  the  soil  was 
demonstrated  by  a  second  planting  on  ground  above  the  original  area, 
with  similar  results.  A  careful  estimate  at  the  end  of  the  season 
showed  wide  differences  in  the  amount  of  injury  sustained,  indicated  as 
follows : 


2  Fawcett,  H.  S.,  Monthly  Bulletin,  California   State  Commission   of  Horti- 
culture, Vol.  2,  p.  613  (1913). 


EXPERIMENTS    WITH    STOCKS    FOR    CITRUS 


277 


Estimated 
percentage 
Stock  of  loss 

Sweet    65% 

Trifoliate   60% 

Sour 40% 

Pomelo    30% 

Kough  lemon  1-  2% 


.J 

--■   -     -   ^ 

, 

0- 

Fig.  1. — Navel  on  sweet  stock. 


STOCKS  FOR  CITRUS 

The  wide  range  of  species  and  varieties  of  citrus  grown  for  com- 
merce, together  with  a  correspondingly  large  number  of  the  genus 
serviceable  as  stocks,  opens  a  wide  field  for  experimentation.  With  a 
marked  variation  in  habit  and  in  reaction  to  environment,  the  problem 
arises  of  determining  some  of  the  factors  governing  optimum  root  con- 
ditions and  the  relative  compatibility  of  the  several  stocks  and  scions. 


278  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  two  stocks  most  commonly  used  in  propagating  citrus  trees 
in  California  are  the  sweet  and  the  sour  orange.  In  addition  to  these, 
pomelo  has  been  employed  to  some  extent,  and  ten  or  twelve  years 
ago  a  considerable  number  of  trees  were  budded  on  trifoliate  orange 
stock.  The  rough  lemon,  so  successfully  employed  in  Florida,  and  the 
lime  are  practically  unknown  as  stocks  in  California.  More  extended 
trials  and  a  careful  study  of  these  lesser  used  stocks  under  varying  con- 
ditions in  this  state  are  greatly  needed  before  definite  recommendations 
for  specific  varieties  and  soils  will  be  fully  warranted. 

Sour  orange  (Citrus  aurantium)  has  proved  generally  satisfactory 
as  a  stock.  It  is  characterized  by  a  well-developed  root  system  which 
spreads  and  penetrates  deeply  into  the  soil;  it  is  quite  resistant  to 
gum  disease,  as  previously  noted,  and  next  to  the  trifoliate  orange  is 
the  hardiest  of  the  citrus  stocks  in  common  use.  When  trees  budded 
on  sour  roots  have  been  severely  frozen,  the  tendency  of  this  stock  to 
send  out  an  abundance  of  new  shoots,  which  may  be  rebudded,  is  an 
advantage. 

As  a  general  rule,  the  quality  of  fruit  produced  on  sour  stock  is  of 
a  high  grade  from  the  time  the  tree  begins  to  bear.  The  yield  for  the 
first  six  or  seven  years  may  be  somewhat  lower  than  that  of  trees  on 
sweet  roots,  according  to  the  experience  of  some  growers,  but  after  that 
period  production  is  said  to  increase  and  to  hold  its  own  with  other 
stocks.  Hume3  is  authority  for  the  statement  that  old  trees  budded 
on  sour  stock  will  sometimes  show  an  increase  in  diameter  of  trunk 
above  the  bud.  Swingle4  confirms  Hume  in  the  experience  that  the 
sour  orange  is  antagonistic  to  the  Satsuma  orange  because  of  some 
unexplained  incompatibility  between  it  and  the  stock  in  question. 

The  sweet  orange  (Citrus  sinensis)  is  doubtless  the  stock  in  great- 
est use  in  California  today  if  we  include  the  considerable  acreage  of 
sweet  seedling  orchards  and  the  sweet  seedling  groves  that  have  been 
rebudded  to  the  standard  varieties.  It  bears  the  reputation  in  Cali- 
fornia, according  to  Mills,5  of  developing  a  more  shallow  root  system 
than  the  sour  orange.  Like  the  latter,  it  sprouts  readily  from  the  trunk 
of  the  tree  below  the  bud-union  when  the  top  has  been  frozen  back, 
but  unlike  sour  stock,  it  is  markedly  susceptible  to  gummosis. 


s  Hume,  II.  IT.,  "Citrus  Fruits  and  Their  Culture,"  p.  199.  Orange  Judd 
Co.,  5th  edition  (1913). 

4  Swingle,  W.  T.,  t '  The  Limitation  of  the  Satsuma  Orange  to  Trifoliate- 
Orange  Stock,"  U.  S.  D.  A.  Bureau  of  Plant  Industry,  Circular  46,  pp.  6-7 
(1909). 

s  Mills,  J.  W.,  "Citrus  Fruit  Culture,"  California  Agricultural  Experiment 
Station  Bulletin  138,  pp.  11-12  (1902). 


EXPERIMENTS   WITH   STOCKS   FOR   CITRUS 


279 


Most  varieties  on  this  root  have  proved  to  be  vigorous  growers  under 
conditions  insuring  freedom  from  this  disease.  As  a  general  rule  sweet 
stock  is  less  hardy  than  sour,  but  more  so  than  pomelo. 

The  pomelo  (Citrus  decumana)  has  in  recent  years  found  vogue  as 
a  stock  for  oranges  and  lemons,  and  although  preferred  in  some  in- 
stances to  sweet  orange  is  not  generally  held  in  as  high  esteem  as  the 


Fig.  2. — Navel  on  trifoliate  stock'. 


sour  root.  Experience  with  pomelo  seedlings  indicate  that  they  may 
be  less  resistant  to  drought  than  any  of  the  stocks  so  far  discussed. 
Some,  however,  have  credited  pomelo  with  the  ability  to  thrive  with 
little  moisture.  It  has  a  vigorous  root  system  and  is  not  considered 
especially  susceptible  to  gummosis.  From  his  examination  of  tree 
roots  Mills6  concluded  that  pomelo  stock  produces  more  fibrous  roots 


e  Mills,  J.  W.,  loc.  tit.,  p.  19. 


280  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

than  sweet  or  sour,  and  that  the  laterals  develop  somewhat  deeper  than 
those  of  the  sweet  orange.  Growth  of  trees  on  pomelo  stock  is  quite 
generally  of  a  rapid  and  vigorous  nature. 

The  rough  lemon  (Citrus  limonum)  is  a  stock  practically  unknown 
in  California  citrus  culture.  The  experience  of  Florida  growers  has 
been  such  as  to  warrant  a  thorough  experimental  test  with  it  in  Cali- 
fornia. This  stock  is  a  seedling  found  growing  wild  in  the  Florida 
woods.  It  has  proved  especially  suited  for  the  conditions  of  some  of 
the  citrus  sections  of  that  state  and  possesses  some  qualities  highly  de- 
sirable for  California  if  it  is  adaptable  to  the  environment  of  this  state. 
According  to  Hume7  it  is  less  hardy  than  either  sweet  orange  or  pomelo, 
and  is  unsuited  to  the  northern  sections  of  Florida.  Its  root  growth 
is  variable,  some  trees  developing  most  of  the  fibrous  roots  near  the 
surface;  the  general  tendency,  however,  is  the  production  of  a  large 
deep  tap-root  and  spreading,  well-distributed  crown-roots.  Its  drought 
resistance  is  great.  Buds  on  this  stock  at  the  Citrus  Experiment  Station 
have  given  a  more  rapid  development  than  on  any  other  root  used. 

This  rapid  growth  is  doubtless  accountable  for  the  fact  that  the 
first  few  crops  from  trees  on  rough  lemon  are  of  poor  quality,  being 
thick  of  skin  and  lacking  in  juice.  After  these  first  few  crops,  how- 
ever, the  undesirable  qualities  do  not  appear. 

Rough  lemon  stock  is  also  credited  with  ability  to  influence  the 
shape  of  the  tree  grown  upon  it,  tending  to  produce  a  tall,  upright 
center. 

Hume  is  again  authority  for  the  statement  that  rough  lemon  stock 
"has  a  marked  influence  on  the  fruitfulness  of  the  Bahia  navel  orange 
in  Florida.  It  is  much  more  prolific  on  rough  lemon  stock  and  with 
the  exception  of  the  trifoliate  orange,  it  is  the  only  stock  which  can  be 
recommended  for  the  variety  in  that  state.  As  a  stock  for  pomeloes 
and  oranges  of  the  mandarin  group,  it  is  preferred  by  some  growers 
to  all  others."8 

Rough  lemon,  like  the  sour  orange,  is  quite  resistant  to  foot-rot,  a 
form  of  gummosis  common  in  Florida. 

The  trifoliate  orange  (Citrus  trifoliata)  was  regarded  with  con- 
siderable favor  as  a  stock  in  California  about  ten  or  fifteen  years  ago, 
but  of  late  has  not  been  held  in  universal  esteem,  owing  to  the  varied 
results  obtained  therewith.  It  is  an  extremely  hardy  tree,  being  in  fact 
the  most  resistant  to  cold  of  any  of  the  genus  known.  According  to 
Swingle,9  it  can  withstand  temperatures  below  zero  Fahrenheit  with- 


7  Hume,  H.  H.,  loc.  cit.,  pp.  200-203. 
s  Hume,  II.  H.,  loc.  cit.,  p.  203. 
9  Swingle,  W.  T.,  loc.  cit.,  p.  5. 


EXPERIMENTS   WITH   STOCKS   FOR   CITRUS 


281 


out  injury.  It  is  deciduous  in  habit  and  has  the  reputation  of  impart- 
ing its  hardiness  in  some  degree  to  the  varieties  of  citrus  budded  upon 
it.  This,  however,  is  a  point  not  fully  established  in  respect  to  Cali- 
fornia conditions.  Its  root  growth  is  characterized  by  the  production 
of  a  great  abundance  of  fibrous  roots  of  good  soil  penetrability.  Unlike 
sweet  or  sour  orange,  the  trifoliate  orange  has  not  a  ready  tendency  to 


Fig.  3. — Valencia  on  sweet  stock. 


develop  buds  from  the  stocks  when  the  tree  upon  it  has  been  frozen  back 
to  the  bud-union.  It  is  generally  recommended  in  Florida  for  clay 
soils  and  those  of  a  heavy  compact  nature,  as  distinct  from  the  drier 
type  of  calcareous  formation. 

An  interesting  development  of  the  citrus  industry  in  the  Gulf  States 
is  the  especial  adaptability  of  Citrus  trifoliata  to  the  Satsuma  orange. 
Swingle10  emphasizes  the  fact  that  for  this  variety  of  the  mandarin  type, 


io  Swingle,  W.  T.,  loc.  tit.,  p.  7. 


282  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

trifoliate  is  the  only  root  that  will  insure  a  successful  tree.  Sweet 
orange,  pomelo,  and  rough  lemon  stocks  have  been  used  with  varying 
success,  and  sour  orange  stock  for  Satsuma  is  a  most  decided  failure. 

Williams11  also  lays  stress  upon  the  necessity  of  using  trifoliate  roots 
for  Satsuma  trees.  The  former,  in  his  opinion,  gives  better  growth  of 
tree  and  yield  of  fruit.  In  addition,  the  fruit  of  trees  of  this  variety 
on  trifoliate  roots  has  been  found  to  be  of  superior  quality  and  ripens 
somewhat  earlier  than  that  grown  on  other  stocks. 

For  the  kumquat  (Citrus  japonica),  the  trifoliate  root  is  also  to  be 
recommended  for  satisfactory  productiveness.  Kumquats  on  sour 
roots  result  in  vigorous  but  practically  barren  trees. 

From  the  brief  description  of  the  different  stocks  and  their  char- 
acteristics just  noted,  the  conclusion  is  naturally  reached  that  despite 
their  possible  use  over  a  considerable  range  of  environment,  a  rather 
definite  relationship  must  exist  between  the  several  stocks  and  factors 
of  soil  and  climate.  The  experience  arising  from  the  development  of 
the  citrus  industry  in  Florida  and  the  Gulf  States  has  in  a  general 
way  outlined  the  geographical  limitations  of  the  several  citrus  stocks. 
Thus  Rolfs12  recommends  for  the  heavy,  moisture-retentive  clays  of 
Louisiana  and  Mississippi  the  trifoliate  stock  as  unqualifiedly  the  best; 
likewise  trifoliate  for  the  eastern  portion  of  northern  Florida,  where 
similar  soil  conditions  prevail.  Where  the  soil  in  this  region  is  of  a 
sandy  type,  some  of  the  less  hardy  stock's  have  proved  satisfactory. 
Pomelo  root  is  given  the  preference  over  sour  stock  for  the  sandy  soils 
of  central  Florida  which  have  a  rather  high  humus  content,  Sour 
orange  is  nevertheless  recognized  as  somewhat  hardier  for  this  region. 

For  southern  Florida  the  rough  lemon  is  considered  the  stock  par 
excellence.  It  is  a  prolific  grower,  subsisting  on  soils  which,  owing  to 
meagerness  of  plant  food,  almost  prohibit  the  use  of  other  stocks. 

For  the  heavier,  non-calcareous  soils  of  the  southern  portion  of  the 
state,  pomelo  and  sour  orange  are  preferred.  They  produce  trees  of 
slower  growth  than  those  on  rough  lemon,  but  yield  fruit  of  better 
quality. 

The  marked  predilection  of  the  Satsuma  orange  for  trifoliate  stock, 
as  already  noted,  makes  this  the  chief  stock  for  the  commercial  plant- 
ings of  the  Gulf  States  where  that  variety  predominates. 


11  Williams,  P.  F.,  "The  Satsuma  Orange,"  Alabama  (College)  Experiment 
Station  Bulletin  157,  pp.  153-155. 

isEolfs,  P.  H.,  "Citrus  Fruit  Growing  in  the  Gulf  States,"  U.  S.  D.  A. 
Farmers'  Bulletin  238,  pp.  35-37  (1906).  "Propagation  of  Citrus  Trees  in  the 
Gulf  States,"  U.  S.  D.  A.  Farmers'  Bulletin  539,  pp.  3-5  (1913). 


EXPERIMENTS   WTTTI    STOCKS   EOR    CITRUS 


283 


Waschka13  also  prefers  trifoliate  to  the  other  stocks  for  Texas  con- 
ditions. According  to  his  experience,  oranges,  pomeloes,  and  lemons  on 
trifoliate  are  not  only  hardier  but  are  more  precocious  and  produce 
their  fruit  early  in  the  season.  Neither  has  he  found  that  the  tri- 
foliate root  dwarfs  the  tree. 

Coit14  recommends  sweet  or  sour  stock  for  the  citrus  regions  of 
Arizona,  and  advises  the  use  of  trifoliate  Avith  reservations  in  view  of 
the  insufficient  and  varied  experience  with  this  root. 


Fig.  4.— Valencia  on  sour  stock. 

The  effect  of  Citrus  trifoliata  upon  the  subsequent  development  of 
the  tree,  i.e.,  its  possible  dwarfing,  is  a  moot  question,  so  far  as  Cali- 
fornia experience  has  gone.    The  observations  that  have  been  reported 

13  Waschka,  S.  A.,  "Report  of  Progress  with  Citrus  Fruits,"  Texas  Agricul- 
tural Experiment  Station  Bulletin  118,  p.  7  (1909). 

14  Coit,  J.  E.,  "Citrus  Culture  in  the  Arid  Southwest,"  Arizona  Agricultural 
Experiment  Station  Bulletin  58,  pp.  303-305. 


284  UNIVERSITY    OF    CALIFORNIA EXPERIMENT   STATION 

to  this  Station  by  growers  are  conflicting.  It  is  impossible  to  draw 
valid  conclusions  therefrom  because  of  the  lack  of  comparable  condi- 
tions or  the  absence  of  definite  data.  There  are  undoubted  examples 
of  dwarfed  citrus  trees  on  trifoliate  roots ;  on  the  other  hand  there  are, 
under  apparently  similar  conditions,  authentic  instances  of  normal 
sized  trees  of  the  same  variety  on  the  stock  in  question.  Hume  takes 
the  following  stand  on  this  point: 

"Many  writers  on  citrus  propagation  have  stated  without  reservation  that 
C.  trifoliata  stock  dwarfs  the  top  worked  upon  it.  Exception  must  be  taken 
to  the  breadth  of  this  statement.  It  is  not  always  true.  Some  varieties  of 
citrus  appear  to  grow  as  rapidly  and  attain  as  large  a  size  when  propagated  on 
trifoliate  orange  stock  as  they  do  on  sour  or  sweet.  It  will  be  found,  however, 
that  the  fruit  borne  on  young  trees  worked  on  trifoliate  orange  is  usually 
superior  in  quality  to  that  borne  on  trees  budded  on  most  other  stocks,  and 
it  may  be  added  that  they  are  decidedly  more  precocious  and  prolific.  "15 

In  addition  to  the  citrus  stock  experiment  at  this  Station,  but  one 
other  experimental  plot,  expressly  laid  out  for  the  determination  of 
data  bearing  on  the  question,  is  known  to  the  writers.  The  work  was 
undertaken  by  Mr.  G.  L.  Taber,  of  Glen  St.  Mary,  Florida.  Hume10 
gives  a  detailed  report  of  the  data  collected,  of  which  the  following  is 
a  general  summary. 

In  March,  1899,  Mr.  Taber  set  out  two  acres  of  virgin  land  to  citrus 
for  a  stock  experiment.  On  this  plot  were  planted  100  orange  and 
pomelo  trees,  four  trees  of  each  of  25  varieties.  Two  trees  of  each  set 
of  four  were  budded  on  trifoliate  stock,  the  other  two  on  sour,  with 
the  exception  of  four  varieties  for  which  sweet  stock  was  used;  the 
stocks  in  each  case  alternating  in  the  rows.  Treatment  of  the  plot  was 
uniform  throughout. 

In  June,  1901,  27  months  after  planting,  all  of  the  trees  on  trifoliate 
stock  were  bearing  fruit,  whereas  13  on  sour  stock  bore  none.  The 
average  number  of  fruits  on  sour  stock  was  13,  on  trifoliate  57.  The 
average  excess  of  height  of  tree  on  sour  over  those  on  trifoliate  stock 
was  one  and  one-quarter  feet,  and  the  average  excess  of  breadth  of 
tree  on  sour  over  trifoliate  stock  was  one  foot. 

At  the  time  that  the  data  for  the  following  year,  1902,  were  taken, 
92  trees  of  23  varieties  were  available  for  comparison.  The  following 
is  a  summary  of  results  three  years  after  planting :  average  height  of 
trees  on  sour  stock,  8  feet  2  inches ;  average  height  of  trees  on  trifoliate 
stock,  6  feet  2  inches ;  average  breadth  of  trees  on  sour  stock,  9  feet  2 


is  Hume,  H.  H.,  loc.  cit.,  pp.  205-207. 
is  Hume,  H.  H.,  loc.  cit.,  pp.  207-216. 


EXPERIMENTS   WITH    STOCKS   FOR    CITRUS  285 

inches;  average  breadth  of  trees  on  trifoliate  stock,  7  feet;  average 
yield  for  two  years  on  sour  stock,  69  fruits  per  tree ;  on  trifoliate  stock, 
93  fruits  per  tree.  Out  of  23  varieties,  two  varieties  of  oranges  and 
one  of  pomelo  averaged  larger  on  trifoliate  than  on  sour  stock  at  the 
end  of  the  third  year,  and  eight  other  varieties  on  trifoliate  stock  com- 
pared favorably  with  sour  stock  in  respect  to  growth. 


Fig.  5. — Valencia  on  pomelo  stock. 

Mr.  Taber  concludes  his  report  of  the  results  up  to  1902  as  follows : 

''One  thing  has  been  fully  determined,  and  that  is  that  all  varieties  come 
into  bearing  at  a  very  early  age  when  budded  upon  Citrus  trifoliata.  Another 
thing  that  is  fully  determined  is  that  the  fruit  from  trees  on  Citrus  trifoliata 
roots  is  fully  equal  in  quality  to  the  same  variety  on  sour  roots,  and  still  an- 
other thiDg  that  experience  has  proven  is  that  the  same  varieties  ripen  earlier 
in  the  season  on  Citrus  trifoliata  than  upon  sour  stock.  The  longevity  of  the 
trees  on  Citrus  trifoliata,  the  ultimate  size  that  they  will  attain,  and  their  com- 
parative value,  in  the  long  run,  with  those  on  sour  stock,  remain  to  be  proven. 


286  UNIVERSITY   OF    CALIFORNIA EXPERIMENT    STATION 

It  is  probable  that  as  the  trees  get  older  there  will  be  more  difference  in  size 
between  trees  on  the  two  stocks  than  is  now  apparent.  This  will  be  the  natural 
consequence  of  the  trees  fruiting  so  heavily  when  young.  This  smaller  ultimate 
size  of  trees  is  a  quality,  whether  advantageous  or  otherwise,  which  everyone 
can  figure  out  for  himself.  It  admits  of  close  planting  and  a  consequent  heavy 
crop  from  a  given  acreage.  It  is  also  probable  that  some  varieties  will  show 
more  affinity  for  the  trifoliate  stock  than  others,  and  that  while  some  varieties 
will  prove  permanently  successful  on  this  stock,  others  may  not.  The  exact 
extent  to  which  extra  hardiness  is  induced  by  using  Citrus  trifoliata  stock  is 
also  more  or  less  conjectural,  but  experience  has  shown  that  the  claim  for 
extra  hardiness  is  well  founded,  with  certain  varieties.  Whether  it  will  prove 
equally  so  with  all  varieties  is  one  of  the  points  that  further  careful  compari- 
sons in  the  test  orchard  must  determine.  The  test  orchard  established  is  one 
of  those  long  time  experiments  in  which  years  must  pass  before  actual  definite 
comparisons  can  be  made  that  will  cover  all  the  points  involved. ' ' 


In  a  letter  received  from  him  about  a  year  and  a  half  ago  (Novem- 
ber 6,  1913),  Mr.  Taber  writes: 

"Brought  up  to  date  and  summarized,  I  might  say  that  all  varieties  budded 
on  Citrus  trifoliata  have  shown  remarkable  fruiting  qualities  both  as  to  the 
earliness  in  which  the  trees  come  into  bearing  and  as  to  their  persistency  in 
fruiting  year  after  year.  This  persistent  fruiting  quality  tends  in  some  varie- 
ties to  reduce  the  size  of  the  trees,  which  is  only  a  natural  consequence.  In 
other  varieties  there  seems  to  be  but  little  difference  in  the  size  of  the  trees 
as  between  Citrus  trifoliata  and  sour  stocks. 

< '  The  fruit  on  Citrus  trifoliata  is  uniformly  of  good  quality,  with  less  coarse 
skinned  rough  fruit  than  that  on  the  same  varieties  on  sour  stock. 

"Bather  heavy,  moderately  damp  land  is  better  suited  to  Citrus  trifoliata 
than  high  sandy  ridges.  Also  the  northern  limit  of  successful  orange  culture 
seems  better  adapted  to  Citrus  trifoliata  than  the  hotter  climates." 


CITRUS  STOCK  EXPERIMENT  AT  THE  RIVERSIDE  STATION 

This  investigation  was  begun  at  the  Citrus  Experiment  Station  in 
April,  1907,  in  accordance  with  plans  prepared  by  Professor  Kalph 
E.  Smith,  assisted  by  Mr.  J.  W.  Mills  and  Mr.  Thomas  Francis  Hunt. 
Until  1912  the  work  was  carried  on  by  Mr.  J.  H.  Norton,  chemist  in 
charge,  later  by  Dr.  J.  Eliot  Coit,  and  since  December,  1912,  by  the 
authors.  The  junior  author  has  also  had  charge  of  the  cultural  opera- 
tions and  the  picking  and  grading  of  the  crops. 

The  trees  comprising  this  experiment  are  planted  in  two  separate 
plots.  The  smaller,  adjacent  to  the  present  Station  building  on  the 
north,  will  in  the  ensuing  discussion  be  designated  as  plot  A.  The 
larger  plot,  B,  is  located  in  the  southeastern  section  of  the  experimental 
citrus  area  east  of  the  present  headquarters. 


EXPERIMENTS   WITH   STOCKS   FOR    CITRUS 


287 


Plot  A  consists  at  present  of  49  trees,  plot  B,  of  164  trees.  The 
varieties  used  are  Washington  navel  and  Valencia  oranges,  and  Eureka 
lemon.  The  stocks  upon  which  these  are  budded  are  sweet  orange,  sour 
orange,  trifoliate  orange,  and  pomelo. 

Differences  in  soil  conditions  necessitate  a  separate  consideration  of 
the  two  plots  and  a  further  division  of  plot  B  into  two  sections.    Both 


•'  • 

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MH 

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,      . 

Fig. 


•Valencia  on  trifoliate  stock. 


main  plots  have  the  same  general  slope  and  drainage.  The  top  soil  is 
dissimilar.  In  A  it  consists  of  a  light,  deep  granitic  loam,  uniform  in 
nature.  That  of  B  is  somewhat  heavier  and  approaches  a  clay  loam 
in  character. 

The  subsoil  in  A  is  deep  and  does  not  differ  substantially  from  the 
overlying  strata ;  that  of  B  is  a  pervious,  sandy  loam  with  wide  varia- 
tions in  depth.    It  is  this  last  named  factor  that  necessitates  a  division 


288 


UNIVERSITY   OF    CALIFORNIA — EXPERIMENT    STATION 


of  B  in  considering  the  data  to  follow.  In  a  portion  of  this  plot  the 
top  soil  is  underlaid  by  hardpan  of  the  most  impervious  character  at 
depths  varying  from  15  inches  to  a  little  over  3  feet.  A  soil  survey 
established  the  respective  deep  and  shallow  areas,  and  the  tree  data 
have  been  grouped  in  accordance  therewith.  Future  reference  will 
therefore  be  made  to  the  two  divisions  of  this  plot  as  B  "deep"  and 
B  "shallow."  It  is  thus  evident  that  for  purposes  of  comparison  there 
are  essentially  three  plots  to  be  considered. 

The  number  of  trees  of  each  variety  on  each  of  the  several  stocks 
in  the  respective  plots  is  as  follows : 

TABLE  I 

Number 
Plot  Variety  Stock  of  trees 

A  *Washmgton   navel  orange,         Sweet  5 

Trifoliate    5 

Valencia  orange,  Sweet  5 

Sour  5 

Pomelo    5 

Trifoliate    5 

Eureka  lemon,  Sweet  5 

Sour  5 

Pomelo    4f 

Trifoliate   5 

Total  : 49 

Bt  Washington   navel   orange,         Sweet  29 

"deep"  Sour  8 

Trifoliate    8 

Valencia  orange,                           Sweet 9 

Sour  4 

Pomelo    7 

Trifoliate    9 

Eureka  lemon,  Sweet 6 

Sour  8 

Pomelo    3 

Trifoliate   8 

Total  99 

*  Plot  A  had  originally  5  navels  on  sour  stock.  Early  in  the  course  of  the 
work  it  was  found  necessary  to  transplant  this  row,  which  excluded  it  from 
the  experiment. 

t  One  of  the  five  lemons  on  pomelo  in  this  plot  failed  to  grow. 

X  A  number  of  trees  of  the  several  varieties  on  different  stocks  in  both 
sections  of  B  have  succumbed  to  cold  or  disease.  This  accounts  for  the  varying 
numbers  in  the  groups. 


B 

1 '  shal- 
low' ' 


EXPERIMENTS   WITH   STOCKS   FOR   CITRUS  289 

Washington   navel   orange,         Sweet  11 

Sour  5 

Trifoliate    6 

Valencia  orange,                           Sweet  6 

Sour  7 

Pomelo    5 

Trifoliate    6 


Fig.  7. — Eureka  on  sweet  stock. 
Eureka  lemon, 


Sweet  

Sour  

Pomelo    ... 
Trifoliate 


Total  65 


The  trees  are  set  in  variety  blocks,  the  respective  stocks  forming 
separate  rows. 


290  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

The  trees  in  all  divisions  were  set  out  in  April,  1907,  as  two-year  old 
buds  of  about  1  inch  caliper,  purchased  from  one  nursery,  and  were 
selected  for  apparent  uniformity  of  condition.  They  have  received  uni- 
form treatment  with  respect  to  cultural  practices.  The  data  for  fertil- 
ization and  cover-crops  follow : 

TABLE  II 

Fertilizer  and  Cover-crops,  1907-1914 

Amount 
Year  Fertilizer  per  tree  Winter  Cover-crop 

1907  Commercial  fertilizer  Vetch  (Vicia  sativa) 
4%  N     8%  P205     4%  K,0 2  lbs. 

1908  Commercial  fertilizer  Vetch  (Vicia  sativa) 
4%  N     8%  P205     4%  K20 2  lbs. 

1909  Commercial  fertilizer  Vetch   (Vicia  sativa) 
4%  N     8%  P203     4%  K20 2  lbs. 

1910  Superphosphate  (18.4%)  Vetch  (Vicia  sativa) 
Available  P205 5  lbs. 

Manure  4  cu.  ft. 

1911  Kaw  rock  phosphate  5  lbs.  Vetch   (Vicia  .sativa) 

Manure   5  cu.  ft. 

1912  Kaw  rock  phosphate  5  lbs.  Vetch  (Vicia  sativa) 

Manure    5  cu.  ft. 

1913  Paw  rock  phosphate  8  lbs.  *Vetch  (Vicia  sativa) 

Manure    0  cu.  ft. 

1914  Paw  rock  phosphate  12  lbs.  Purple  Vetch   (Vicia 

Manure   5  cu.  ft.  atropurpurea) 

*  Melilotus  indica  took  the  place  of  vetch  on  Plot  A  for  the  year  1913. 


The  data  derived  from  the  experiment  to  date  are  concerned  with 
three  of  the  more  important  considerations  bearing'  on  the  effect  of 
stock  upon  scion.  These  are  yield  and  commercial  quality  of  fruit,  and 
vigor  of  tree.  The  manner  of  obtaining  the  first  of  these  is  self- 
evident  ;  quality  in  this  case  does  not  refer  to  intrinsic  properties,  such 
as  flavor,  texture  of  pulp,  or  percentage  of  "rag,"  but  relates  to  mar- 
ket standards,  which  take  into  account  size  of  fruit,  texture  of  rind, 
and  freedom  from  blemishes. 

For  determining  the  data  on  these  points,  the  fruit  of  each  tree 
was  picked  separately.  It  was  then  separated  according  to  market  sizes 
by  a  mechanical  grader,  and  every  fruit  then  carefully  examined  for 
quality  and  for  defects  of  rind. 

The  weight  of  every  tree  crop  was  also  noted,  inasmuch  as  the 


EXPERIMENTS   WITH   STOCKS   EOR   CITRUS 


201 


average  weight  and  number  of  fruits  will  serve  the  experienced  grower 
in  a  general  way  as  an  index  of  the  average  size  of  the  fruits. 

Growth  of  trees  in  terms  of  volume  was  taken  as  an  index  of  vigor. 
The  methods  used  in  the  determinations  will  be  considered  later. 

The  average  yield  of  fruit  per  tree,  both  in  number  and  weight, 
has  been  based  on  the  number  of  trees  in  each  division  and  the  number 
of  actual  crops  produced  by  the  same,  instead  of  the  actual  number  of 


Fig.  8. — Eureka  on  sour  stock. 

seasons.  This  is  believed  to  be  the  fairest  basis  of  comparison,  in  view 
of  the  fact  that  although  a  majority  of  the  sections  have  yielded  four 
crops,  from  1910  to  1914,  inclusive,  six  have  borne  fruit  in  but  three. 
Especially  is  this  the  case  with  Eureka  lemon,  the  failure  of  the  1913 
crop  of  this  fruit  in  plot  B,  and  an  almost  negligible  yield  in  A,  being 
undoubtedly  due  to  the  severe  cold  wave  of  January,  1912. 

The  crop  year  coincides  with  that  established  by  the  California 
Fruit  Growers'  Exchange,  which  begins  November  first. 


202 


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EXPERIMENTS   WITH   STOCKS   FOR   CITRUS 


295 


A  study  of  the  figures  above  shows  no  evidence  of  significant  dif- 
ferences in  the  quality  of  oranges  produced  by  the  respective  varieties 
on  the  several  stocks.  The  percentage  of  each  grade  for  any  one  variety 
in  the  same  section  appears  to  have  no  relation  to  the  particular  root ; 
seldom  are  such  differences  greater  than  4  or  5  per  cent.  The  one 
exception  is  Valencia  on  pomelo,  B  "shallow,"  where  "fancy"  fruit 


Fig.  9. — Eureka  on  pomelo  stock. 


of  trees  on  trifoliate  falls  considerably  below  the  same  grade  from 
trees  on  sweet  and  pomelo  stock.  It  is  not  much  below  the  grade  per 
cent  of  fruit  on  sour  stock. 

Quality  differences  are  more  evident  in  the  case  of  the  lemon.  Here 
we  may  note  that  although  the  respective  grades  run  closely  to  each 
other  for  sweet,  sour,  and  pomelo  stocks,  they  fall  off  noticeably  in  each 
block  when  trifoliate  root  has  been  used.    There  is  a  lower  percentage 


296  UNIVERSITY    OF    CALIFORNIA EXPERIMENT    STATION 

of  fancy  and  choice,  and  a  relatively  higher  amount  of  standard  and 
cull  fruit.  Here  again  there  is  an  exception  to  this  general  statement 
in  the  case  of  pomelo  in  B  "shallow,"  where  the  percentage  of  "fancy" 
is  less  than  that  from  trees  on  trifoliate.  This  exception,  as  well  as 
the  one  indicated  for  Valencia  orange,  is  not  entitled  to  much  weight, 
inasmuch  as  the  "shallow"  section  of  B  represents  abnormal  conditions. 

The  yield  of  fruit,  considered  either  from  the  values  of  number  or 
weight,  shows  more  striking  deviations.  In  plot  A  the  yield  of  navels 
on  sweet  stock  is  almost  double  that  of  the  variety  on  trifoliate.  In 
both  sections  of  B,  on  the  other  hand,  the  two  roots  have  produced 
practically  equal  results,  and  both  are  exceeded  by  sour  stock  by  a 
wide  margin. 

Different  rank  is  again  evident  in  the  case  of  the  Valencia  orange. 
In  Plot  A  sweet  stock  is  easity  first,  pomelo  a  close  second,  trifoliate 
not  far  behind  the  latter,  and  sour  stock  considerably  lower  than  any  of 
the  others.  The  order  is  almost  reversed  in  the  "deep"  section  of  B, 
where  trifoliate  exceeds  sour,  pomelo  is  third,  and  sweet  stock  is  here 
lowest  of  all  in  yield.    Still  another  order  obtains  in  b  "shallow." 

For  a  comparison  of  the  varying  effect  of  the  stock  upon  the 
growth  of  the  tree,  the  volume  of  the  top  was  taken  as  a  sufficiently 
close  index.  This  was  ascertained  with  the  regulation  fumigation  tent 
used  in  combatting  the  scale  insect  pests  of  the  citrus  regions.  The 
greatest  circumference  of  the  tree  under  the  tent  and  the  measure- 
ment "over"  it  were  taken.  The  latter  is  the  linear  expression  of  the 
periphery  of  that  portion  of  a  vertical  plane  passing  through  the  high- 
est point  of  the  tree  and  extending  from  the  surface  of  the  ground 
on  one  side  to  the  other.  Although  the  volume  obtained  in  this  manner 
is  not  an  exact  measurement  of  the  tree  volume,  since  it  includes  the 
variable  space  between  the  lowest  growth  of  the  tree  and  the  surface 
of  the  earth,  it  is  nevertheless  sufficiently  close  for  purposes  of  com- 
parison. 

By  substitution  of  the  dimensions  above  referred  to  in  WoglumV7 
formula,  the  volume  under  the  tent  is  obtained  in  cubic  feet. 


iTWoglum,  R.   S.,   "Fumigation   of  Citrus   Trees,"   U.   S.   D.   A.,  Bureau   of 
Entomology,  Bulletin  90,  Part  I,  p.  28. 

C2     /0        C(3tt  —  4)\ 
Volume  in  cubic  feet  =  - —    I  —  —  • ^__ J 

C  =  circumference  of  tree, 

0  =  distance    over   top   of   tree. 


EXPERIMENTS   WITH   STOCKS   FOR    CITRUS 


297 


TABLE  TV 

Average  Volumes  of  Trees   (Cubic  Feet)   in   1914 


Washington    Navel    Orange 


Valencia  Orange 


Eureka  Lemon 


Plot 

A 

B  Deep 

B  Shallow 

Sweet 

1061 

583 

420 

Sour 

551 

440 

Trifoliate 

695 

525 

341 

Sweet 

1128 

790 

676 

Sour 

788 

592 

499 

Pomelo 

1112 

730 

577 

Trifoliate 

818 

786 

335 

Sweet 

1434 

998 

807 

Sour 

1032 

698 

713 

Pomelo 

1270 

693 

378 

Trifoliate 

199 

164 

167 

Fig.  10. — Eureka  on  trifoliate  stock. 


298  UNIVERSITY    OP    CALIFORNIA — EXPERIMENT    STATION 

The  data  on  size  of  trees  is  of  especial  interest  in  view  of  the  repu- 
tation that  Citrus  trifoliata  stock  has  gained  for  dwarfing  the  tree.18 

In  plot  A  navels  on  trifoliate  average  practically  one-third  smaller 
than  the  trees  on  sweet  stock.  In  B  "deep"  the  differences  between 
the  three  stocks  are,  for  all  practical  purposes,  negligible. 

Valencias  in  plot  A  have  attained  almost  equal  size  on  sweet  and 
pomelo  roots,  and  trifoliate  has  produced  a  larger  tree  than  sour  stock. 
In  B  "deep,"  on  the  other  hand,  Valencia  on  trifoliate  is  practically 
as  large  as  the  variety  on  sweet  root,  the  difference  in  average  volume 
amounting  to  but  4  cubic  feet.  The  use  of  sour  stock  has  here  resulted 
in  the  smallest  growth,  pomelo  stock  exceeding  it. 

The  most  striking  effect,  however,  is  to  be  seen  in  the  Eureka  lemon 
sections  of  both  plots.  In  A,  sweet  stock  has  produced  the  largest  tree, 
pomelo  stock  ranks  second,  and  sour  stock  third.  The  trifoliate  root 
has  here  produced  a  most  remarkable  dwarfing  of  the  tree.  Almost  the 
same  relative  positions  and  values  hold  in  the  deep  section  of  B.  In  this 
case  sour  stock  slightly  outranks  pomelo.  The  values  for  B  "shallow" 
are  not  consistent  with  those  just  considered,  and  this  is  undoubtedly 
due  to  the  irregular  and  wide  deviations  in  the  depth  of  the  soil  over 
the  hardpan. 

!8  In  numerous  instances  following  the  freeze  of  January,  1913,  the  slightly 
increased  hardiness  of  varieties  grown  on  trifoliate  root  stocks  was  observed. 
This  increase  in  hardiness  was  not  sufficiently  great  to  be  of  importance  in 
California  in  view  of  the  unsatisfactory  growfh  that  our  principal  varieties 
apparently  make  on  this  stock,  but  is  a  factor  that  may  be  of  importance  in 
other  localities. 

The  following  notes  recorded  by  Prof.  E.  S.  Vaile  of  the  Citrus  Experiment 
Station  are  also  of  interest  in  considering  the  effect  of  trifoliate  stock. 

Notes  on  trifoliate  Boot  Stock.    January  to  May,  1915 
' '  Morris  Smith  Grove,  West  Orange-Thorpe  Avenue,  Santa  Ana,  just  west  of 
Orange  Thorpe  School. 
1 '  Six  year  old  Valencias  on  trifoliate  root — have  seemingly  been  well  cared 
for.     Are  very  badly  stunted,  growing  close  to  ground  without  character- 
istic long  straight  leaders  of  normal  Valencias. 
"Bishop  Property  on  Fairhaven  Avenue,  Orange,  Calif. 

"Four  or  five  Valencia  resets  six  years  old  on  trifoliate  root  stock.     Have 
made  not  to  exceed  one-half  the  growth  of  other  trees  on  sweet  root  set 
at  the  same  time. 
1 '  Eancho  Sespe,  Fillmore. 

"A  considerable  block  of  Eureka  lemon  trees  planted  on  trifoliate  root. 
Difference  in  size  between  these  trees  and  adjoining  trees  on  sweet  or 
sour  root  was  very  marked  at  five  years  of  age.  Foliage  was  lighter  in 
color  and  less  vigorous  on  trifoliate.  Trees  finally  replaced  with  new 
ones  on  sour  root. 
"Douglas  Grove,  East  Badillo  Avenue,  Covina. 

"Twenty-five  year  old  Valencia  Grove,  medium  heavy  soil.  Most  of  trees 
are  very  large.  Block  of  about  50  trees  near  center  of  grove  decidedly 
stunted  in  comparison.  No  apparent  differences  in  soil  to  depth  of  4 
feet.  These  smaller  trees  are  on  trifoliate  stock,  while  others  are  on 
either  sweet  or  sour  orange. 

"H.  J.  Webber." 


EXPERIMENTS   WITH   STOCKS   FOR   CITRUS 


299 


DISCUSSION 


A  consideration  of  the  table  of  comparative  yields  makes  it  clear 
that  these  data  are  purely  indicative  and  in  no  sense  proper  as  a  basis 
for  definite  conclusions.  The  yields  indicated  are  in  all  cases  extremely 
small,  even  for  average  values.    This  is  of  course  primarily  due  to  the 


-Valencia  on  trifoliate  stock,  showing  effect  of  scion  on  stock 
(increased  diameter  of  trunk  below  bud-union). 


fact  that  the  figures  are  those  of  the  first  years  of  production  in  the  life 
of  the  trees ;  the  very  low  yield  of  the  first  year  or  two  has  naturally 
reduced  the  average  for  the  five  years  to  a  considerable  degree.  Al- 
lowance must  also  be  made  for  the  severe  cold  waves  of  December,  1911, 
and  January,  1913,  which  may  have  had  their  effects  upon  the  suc- 
ceeding bloom  and  crops.  This  is  undoubtedly  true  in  the  case  of  the 
lemon  yields. 


1300 


UNIVERSITY    OP    CALIFORNIA EXPERIMENT    STATION 


It  is  evident,  however,  that  there  is  no  ground  at  present  for  recom- 
mending any  particular  stock  for  increased  yield  or  quality  of  fruit 
of  either  of  the  oranges  or  the  lemon  used  in  these  experiments.  Such 
evidence  could  scarcely  be  expected  at  the  end  of  the  first  half -decade  of 
an  experiment  the  nature  of  which  demands  an  accumulation  of  many 
years '  data  as  a  prerequisite  for  sound  conclusions.    Assuming  a  future 


j 

■        I 

fl               B      1                B 

1 

Fig.  3  2. — Eureka  lemon  on  trifoliate  stock,  showing  effect  of  scion  on 
stock  (increased  diameter  of  trunk  below  bud-union)  in  addition  to  dwarf- 
ing effect  of  stock  on  scion. 


increased  annual  production  consistent  with  the  increasing  age  of  the 
trees,  we  may  properly  look  for  more  differences  between  the  yields  of 
trees  on  different  stocks  in  the  averages  of  the  next  five  or  ten  years, 
if  such  differences  are  to  become  evident  under  the  conditions  of  the 
experiment.  Moreover,  any  factor  making  for  reduction  in  yield  or 
quality  of  fruit,  or  in  size  of  tree,  which  may  be  associated  with  early 


EXPERIMENTS    WITH   STOCKS   FOR   CITRUS  301 

growth — a  characteristic  accredited  to  sour  stock,  for  example — should 
disappear  in  the  future  records. 

Conclusions  regarding  the  superiority  of  any  one  stock  for  its  effect 
upon  growth  are  likewise  wholly  unwarranted  at  present.  Attention 
is  called  to  the  fact  that  the  behavior  of  the  orange,  both  navel  and 
Valencia,  has  varied  on  the  two  plots  in  regard  to  the  size  of  trees  on 
trifoliate  and  the  other  stocks,  and  that  in  no  case  has  the  use  of  the 
former  effected  a  marked  or  undesirable  dwarfing.  (Figures  1-6). 
Valencia  has  been  particularly  thrifty  on  the  trifoliate  root.  Indeed, 
the  most  striking  fact  of  the  experiment  evident  to  date  is  that  Citrus 
trifoliata  per  se  does  not  necessarily  dwarf  all  species  or  varieties 
budded  upon  it.  Where  such  dwarfing  occurs,  it  is  highly  probable 
that  conditions  of  environment  are  largely  responsible.  Further  and 
more  extensive  investigation  of  these  stocks  on  a  number  of  widely 
different  soil  types  is  essential  for  a  final  answer  to  this  question. 

On  the  other  hand,  the  dwarfing  of  Eureka  lemon  on  trifoliate  root 
has  been  so  extreme  and  so  consistent  that  we  may  justifiably  regard 
it  as  highly  undesirable  for  that  fruit.     (Figures  7-10). 

A  point  of  interest  in  connection  with  the  use  of  Citrus  trifoliata 
as  a  stock  is  the  characteristic  increase  of  the  diameter  of  the  trunk 
below  the  bud-union.  (Figure  11).  This  seems  to  be  a  constant  result 
of  the  union  of  the  comparatively  small  root  system,  accustomed  norm- 
ally to  develop  a  small  tree,  with  the  bud  of  a  species  of  large  growth. 
Such  evident  root  stimulus  appears  in  the  trifoliate  stock  irrespective 
of  the  variety  of  orange  or  lemon  budded  upon  it.  In  the  case  of  the 
latter  the  increase  of  trunk  persists  even  when  the  counter  effect  results 
in  the  dwarfing  of  the  tree.     (Figure  12). 

It  must  be  pointed  out  in  conclusion  that  this  Station,  despite  the 
favorable  results  so  far  obtained  with  the  trifoliate  orange  as  a  stock  for 
oranges,  cannot  recommend  it  at  present,  in  view  of  the  fact  that  well- 
defined  cases  are  known  where  its  use  has  been  injurious.  In  the 
absence  of  further  knowledge,  with  the  limited  data  at  hand,  the  use 
of  either  sweet  or  sour  stock  is,  therefore,  to  be  recommended  on 
grounds  of  wide  and  successful  experience  of  California  growers. 
Where  conditions  are  favorable  for  the  development  of  gummosis,  sour 
stock  should  be  given  the  preference. 


STATION  PUBLICATIONS  AVAILABLE  FOR  DISTRIBUTION 


REPORTS 

Adaptation, 


and   Grafting.      Appendix  to    Viticultural 


18D7.      Resistant    Vines,    their   Selection, 

Report  for  1896. 
L902.      Report  of  the  Agricultural  Experiment  Station   for    1898-1901. 

1903.  Report  of  the  Agricultural  Experiment  Station  for   1901-03. 

1904.  Twenty-second  Report  of  the  Agricultural   Experiment  Station   for  1903-04. 

1914.  Report  of  the   College  of   Agriculture   and  the  Agricultural   Experiment    Station, 

1913-June,   1914. 

1915.  Report   of  the   College  of   Agriculture   and   the   Agricultural   Experiment   Station, 

1914-June,   1915. 


July, 
July, 


No. 


169. 
174. 
178. 
184. 

185. 

195. 

197. 


198. 
203. 

207. 
208. 
212. 
213. 
216. 


220. 
225. 
227. 
230. 
234. 
241. 


Observations  on  Some  Vine  Diseases 
in   Sonoma  County. 

Tolerance  of  the  Sugar  Beet  for  Alkali. 

A  New  Wine-Cooling  Machine. 

Mosquito   Control. 

Report  of  the  Plant  Pathologist  to 
July   1,    1906. 

Report  of  Progress  in  Cereal  Investi- 
gations. 

The  California   Grape  Root-worm. 

Grape  Culture  in  California ;  Improved 
Methods  of  Wine-making  ;  Yeast  from 
California  Grapes. 

The   Grape   Leaf-Hopper. 

Report  of  the  Plant  Pathologist  to 
July   1,    1909. 

The  Control  of  the  Argentine  Ant. 

The  Late  Blight  of  Celery. 

California  White  Wheats. 

The   Principles  of  Wine-making. 

A  Progress  Report  Upon  Soil  and  Cli- 
matic Factors  Influencing  the  Com- 
position of  Wheat. 

Dosage  Tables. 

Tolerance  of  Eucalyptus  for  Alkali. 

Grape  Vinegar. 

Enological   Investigations. 

Red  Spiders  and  Mites  of  Citrus  Trees. 

Vine  Pruning  in  California,  Part  I. 


BULLETINS 
No. 
242. 
244. 
246. 
248. 


249. 
250. 
251. 


252. 
253. 

254. 
255. 
256. 
257. 
258. 
261. 

262. 

263. 

264. 
265. 
266. 

267. 


Humus   in   California   Soils. 
Utilization  of  Waste  Oranges. 
Vine   Pruning  in   California,    Part   II. 
The   Economic  Value  of  Pacific   Coast 

Kelps. 
Stock-Poisoning  Plants  of  California. 
The  Loquat. 

Utilization  of  the  Nitrogen  and  Organic- 
Matter    in    Septic    and    Imhoff    Tank 

Sludges. 
Deterioration  of  Lumber. 
Irrigation    and    Soil   Conditions    in   the 

Sierra   Nevada   Foothills,    California. 
The  Avocado  in  California. 
The  Citricola  Scale. 
Value  of  Barlev  for  Cows  Fed  Alfalfa. 
New  Dosage  Tables. 
Mealy  Bugs  of  Citrus  Trees. 
Melaxuma    of    the    Walnut,     "Juglans 

regia." 
Citrus   Diseases   of   Florida   and   Cuba 

Compared  with  Those  of  California. 
Size  Grade  for  Ripe  Olives. 
The  Calibration  of  the  Leakage  Meter. 
Cottony  Rot  of  Lemons  in  California. 
A  Spotting  of  Citrus  Fruits  Due  to  the 

Action  of  Oil  Liberated  from  the  Rind. 
Experiments  with  Stocks  for  Citrus. 


No. 
65. 
69. 

70. 

76. 

80. 


83. 
100. 
106. 

107. 

108. 
109. 


110. 
111. 

113. 
114. 
115. 
117. 

118. 
119. 
121. 

122. 


CIRCULARS 

No. 


The  California   Insecticide  Law. 

The    Extermination    of    Morning-Glory. 

Observations  on  the  Status  of  Corn 
Growing  in   California. 

Hot   Room    Callusing. 

Boys'    and   Girls'    Clubs. 

The  Common  Ground  Squirrels  of 
California. 

Potato  Growing  Clubs. 

Pruning  Frosted  Citrus   Trees. 

Directions  for  Using  Anti-Hog  Cholera 
Serum. 

Spraying  Walnut  Trees  for  Blight  and 
Aphis    Control. 

Grape  Juice. 

Community  or  Local  Extension  Work 
by  the  High  School  Agricultural  De- 
partment. 

Green   Manuring  in  California. 

The  Use  of  Lime  and  Gypsum  on  Cali- 
fornia Soils. 

Correspondence  Courses  in  Agriculture. 

Increasing  the  Duty  of  Water. 

Grafting  Vinifera  Vineyards. 

The  Selection  and  Cost  of  a  Small 
Pumping  Plant. 

The  County  Farm  Bureau. 

Winery  Directions. 

Some  Things  the  Prospective  Settler- 
Should  Know. 

The  Management  of  Strawberry  Soils 
in  Pajaro  Valley. 


124. 
125. 
126. 
127. 
128. 
129. 
130. 
131. 
132. 

133. 
134. 
135. 
136. 
137. 
138. 
139. 


140. 


142. 
143. 


144. 
145. 


146. 


Alfalfa   Silage  for  Fattening   Steers. 

Aphids  on  Grain  and  Cantaloupes. 

Spraying  for  the  Grape  Leaf  Hopper. 

House  Fumigation. 

Insecticide   Formulas. 

The  Control  of  Citrus  Insects. 

Cabbage   Growing  in   California. 

Spraying  for  Control  of  Walnut  Aphis. 

When  to  Vaccinate  against  Hog 
Cholera. 

County  Farm  Adviser. 

Control  of  Raisin   Insects. 

Official  Tests  of  Dairy  Cows. 

Melilotus   Indica. 

Wood  Decay  in  Orchard  Trees. 

The  Silo  in  California  Agriculture. 

The  Generation  of  Hydrocyanic  Acid 
Gas  in  Fumigation  by  Portable  Ma- 
chines. 

The  Practical  Application  of  Improved 
Methods  of  Fermentation  in  Califor- 
nia Wineries  during  1913  and  1914. 

Standard  Insecticides  and  Fungicides 
versus   Secret  Preparations. 

Practical  and  Inexpensive  Poultry  Ap- 
pliances. 

Control  of  Grasshoppers  in  Imperial 
Valley. 

Oidium  or  Powdery  Mildew  of  the  Vine. 

Suggestions  to  Poultrymen  concerning 
Chicken  Pox. 

Jellies  and  Marmalades  from  Citrus 
Fruits. 


